In general, a digital signal is a signal used for transmission of information which utilizes a two value logic of a high or the low level of a voltage, or existence or non-existence of a current. In transmitting continuous information utilizing such a digital signal a pulse width modulation signal (hereinafter referred to as a "PWM signal") is widely used thereby to simplify the receiver. This is because an analog signal simply can be easily obtained from the PWM signal by integrating the pulses which are transmitted continuously, that is, by passing the same through a low pass filter.
When the PWM signal is used, the receiver has a simplified construction as described above, but it becomes necessary to read out the information to be transmitted at the period of the PWM signal at the transmitter's side. This is ineffective in view of the advantageous utilization of the transmission path because a PWM signal always occupies a transmission path. In other words, in order to transmit a plurality of analog information signals, that number of transmission paths are required.